Adding "units" to USPS Cluster Mailbox

[Daniel Chang]

I've done a search but unable to find an answer.  Hoping someone knows.

I have a 12 unit USPS cluster mailbox for my property.  My property only has 10 units.   Can I just relabel the unused 2 boxes as say "unit 11" and "unit 12" and get mail delivered?  This would be quite useful if a tenant would like to pay more for an extra mailing address.  

[Matt R.]

Disclaimer: I don't work for the post office.  I have worked with some mailing-list stuff before.

I think the post office has an idea in their computer of what addresses are "good" or not.  In other words, they know that there really is a house at 104 Oak Street, and that there really is a house next door at 108 Oak Street, but that there is no house at 106 Oak Street.  That way, if somebody mails a letter from across the country to 106 Oak Street, they "know" it's a bad address right away, and can return it to the sender after it first gets scanned, usually close to where it was mailed.  They avoid shipping the mail across the country, having the local post office say "nope, bad address", and then shipping it back to the sender.

I am pretty sure multiple units at an address work the same way; the post office "knows" that 112 Oak Street, Unit 1 through 10 are valid, but no other unit numbers are valid.  Same reason - they can kick back invalid addresses sooner.

The ZIP+4 code localizes delivery to a few single-family houses or a few apartments.  There is an additional two-digit code inside that bar code at the bottom of the mail that narrows it down to a single house, apartment, unit, or whatever.  The post office optically scans the human-readable address and then prints the bar code on the envelope; if it "knows" there is no Unit 11 or 12, it will kick back the mail to be returned to sender, rather than printing the bar code and sending it on to be delivered.

There may be a way to get them to officially add the "extra" units to your property's address, but I have had limited success doing anything online with USPS.  It usually works better to go to the post office yourself and ask.  If you can, start with the post office that actually handles the mail for your property - sometimes that's the closest one and sometimes not.  If you happen to be at the property when the mailman comes by, ask him or her which post office it is.